Mutexes¶
A mutex is the basic synchronization method used within Traffic Server to protect data from simultaneous access by multiple threads. A mutex acts as a lock that protects data in one program thread from being accessed by another thread.
The Traffic Server API provides two functions that attempt to access and
lock the data: TSMutexLockTry()
and TSMutexLock()
.
TSMutexLock
is a blocking call - if you use it, you can slow
Traffic Server performance because transaction processing pauses until
the mutex is unlocked. It should be used only on threads created by the
plugin TSContThreadCreate
. Never use it on a continuation handler
called back by the Cache, Net, or Event Processor. Even if the critical
section is very small, do not use it. If you need to update a flag, then
set a variable and/or use atomic operations. If TSMutexLock()
is used
in any case other than the one recommended above, then the result will
be a serious performance impact.
TSMutexLockTry
, on the other hand, attempts to lock the mutex
only if it is unlocked (i.e., not being used by another thread). It
should be used in all cases other than the above mentioned
TSMutexLock
case. If the TSMutexLockTry
attempt fails, then you
can schedule a future attempt (which must be at least 10 milliseconds
later).
In general, you should use TSMutexLockTry
instead of
TSMutexLock
.
TSMutexLockTry
is required if you are tying to lock Traffic Server internal or system resources (such as the network, cache, event processor, HTTP state machines, and IO buffers).TSMutexLockTry
is required if you are making any blocking calls (such as network, cache, or file IO calls).TSMutexLock
might not be necessary if you are not making blocking calls and if you are only accessing local resources.
The Traffic Server API uses the TSMutex
type for a mutex. There are
two typical uses of mutex. One use is for locking global data or data
shared by various continuations. The other typical usage is for locking
data associated with a continuation (i.e., data that might be accessed
by other continuations).
Locking Global Data¶
The blacklist-1.c sample plugin implements a mutex that locks global
data. The blacklist plugin reads its blacklisted sites from a
configuration file; file read operations are protected by a mutex
created in TSPluginInit()
. The blacklist-1.c code uses
TSMutexLockTry()
instead of TSMutexLock()
. For more detailed
information, see the blacklist-1.c code;
start by looking at the TSPluginInit()
function.
General guidelines for locking shared data are as follows:
- Create a mutex for the shared data with
TSMutexCreate()
. - Whenever you need to read or modify this data, first lock it by
calling
TSMutexLockTry()
; then read or modify the data. - When you are done with the data, unlock it with
TSMutexUnlock()
. If you are unlocking data accessed during the processing of an HTTP transaction, then you must unlock it before callingTSHttpTxnReenable()
.
Protecting a Continuation’s Data¶
You must create a mutex to protect a continuation’s data if it might be accessed by other continuations or processes. Here’s how:
- Create a mutex for the continuation using
TSMutexCreate
.For example:TSMutex mutexp; mutexp = TSMutexCreate ();
- When you create the continuation, specify this mutex as the continuation’s mutex.For example:
TSCont contp; contp = TSContCreate (handler, mutexp);
If any other functions want to access contp
’s data, then it is up to
them to get contp
’s mutex (using, for example, TSContMutexGet
)
to lock it. For usage, ssee the sample Protocol plugin.
How to Associate a Continuation With Every HTTP Transaction¶
There could be several reasons why you’d want to create a continuation for each HTTP transaction that calls back your plugin.
Some potential scenarios are listed below.
- You want to register hooks locally with the new continuation instead of registering them globally to the continuation plugin.
- You want to store data specific to each HTTP transaction that you might need to reuse across various hooks.
- You’re using APIs (like
TSHostLookup
) that will call back the continuation with a certain event.
How to Add the New Continuation¶
A typical way of adding the new continuation is by registering the
plugin continuation to be called back by HTTP transactions globally when
they reach TS_HTTP_TXN_START_HOOK
. Refer to the example below, which
uses a transaction-specific continuation called txn_contp
.
void TSPluginInit(int argc, const char *argv[])
{
/* Plugin continuation */
TSCont contp;
if ((contp = TSContCreate (plugin_cont_handler, NULL)) == TS_ERROR_PTR) {
LOG_ERROR("TSContCreate");
} else {
if (TSHttpHookAdd (TS_HTTP_TXN_START_HOOK, contp) == TS_ERROR) {
LOG_ERROR("TSHttpHookAdd");
}
}
}
In the plugin continuation handler, create the new continuation
txn_contp
and then register it to be called back at
TS_HTTP_TXN_CLOSE_HOOK
:
static int plugin_cont_handler(TSCont contp, TSEvent event, void *edata)
{
TSHttpTxn txnp = (TSHttpTxn)edata;
TSCont txn_contp;
switch (event) {
case TS_EVENT_HTTP_TXN_START:
/* Create the HTTP txn continuation */
txn_contp = TSContCreate(txn_cont_handler, NULL);
/* Register txn_contp to be called back when txnp reaches TS_HTTP_TXN_CLOSE_HOOK */
if (TSHttpTxnHookAdd (txnp, TS_HTTP_TXN_CLOSE_HOOK, txn_contp) == TS_ERROR) {
LOG_ERROR("TSHttpTxnHookAdd");
}
break;
default:
TSAssert(!"Unexpected Event");
break;
}
if (TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE) == TS_ERROR) {
LOG_ERROR("TSHttpTxnReenable");
}
return 0;
}
Remember that the txn_contp
handler must destory itself when the
HTTP transaction is closed. If you forget to do this, then your plugin
will have a memory leak.
static int txn_cont_handler(TSCont txn_contp, TSEvent event, void *edata)
{
TSHttpTxn txnp;
switch (event) {
case TS_EVENT_HTTP_TXN_CLOSE:
txnp = (TSHttpTxn) edata;
TSContDestroy(txn_contp);
break;
default:
TSAssert(!"Unexpected Event");
break;
}
if (TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE) == TS_ERROR) {
LOG_ERROR("TSHttpTxnReenable");
}
return 0;
}
How to Store Data Specific to Each HTTP Transaction¶
For the example above, store the data in the txn_contp
data
structure - this means that you’ll create your own data structure. Now
suppose you want to store the state of the HTTP transaction:
typedef struct {
int state;
} ContData;
You need to allocate the memory and initialize this structure for each
HTTP txnp
. You can do that in the plugin continuation handler when
it is called back with TS_EVENT_HTTP_TXN_START
static int plugin_cont_handler(TSCont contp, TSEvent event, void *edata)
{
TSHttpTxn txnp = (TSHttpTxn)edata;
TSCont txn_contp;
ContData *contData;
switch (event) {
case TS_EVENT_HTTP_TXN_START:
/* Create the HTTP txn continuation */
txn_contp = TSContCreate(txn_cont_handler, NULL);
/* Allocate and initialize the txn_contp data */
contData = (ContData*) TSmalloc(sizeof(ContData));
contData->state = 0;
if (TSContDataSet(txn_contp, contData) == TS_ERROR) {
LOG_ERROR("TSContDataSet");
}
/* Register txn_contp to be called back when txnp reaches TS_HTTP_TXN_CLOSE_HOOK */
if (TSHttpTxnHookAdd (txnp, TS_HTTP_TXN_CLOSE_HOOK, txn_contp) == TS_ERROR) {
LOG_ERROR("TSHttpTxnHookAdd");
}
break;
default:
TSAssert(!"Unexpected Event");
break;
}
if (TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE) == TS_ERROR) {
LOG_ERROR("TSHttpTxnReenable");
}
return 0;
}
For accessing this data from anywhere, use TSContDataGet:
TSCont txn_contp;
ContData *contData;
contData = TSContDataGet(txn_contp);
if (contData == TS_ERROR_PTR) {
LOG_ERROR("TSContDataGet");
}
contData->state = 1;
Remember to free this memory before destroying the continuation:
static int txn_cont_handler(TSCont txn_contp, TSEvent event, void *edata)
{
TSHttpTxn txnp;
ContData *contData;
switch (event) {
case TS_EVENT_HTTP_TXN_CLOSE:
txnp = (TSHttpTxn) edata;
contData = TSContDataGet(txn_contp);
if (contData == TS_ERROR_PTR) {
LOG_ERROR("TSContDataGet");
} else {
TSfree(contData);
}
TSContDestroy(txn_contp);
break;
default:
TSAssert(!"Unexpected Event");
break;
}
if (TSHttpTxnReenable(txnp, TS_EVENT_HTTP_CONTINUE) == TS_ERROR) {
LOG_ERROR("TSHttpTxnReenable");
}
return 0;
}
Using Locks¶
You do not need to use locks when a continuation has registered itself
to be called back by HTTP hooks and it only uses the HTTP APIs. In the
example above, the continuation txn_contp
has registered itself to
be called back at HTTP hooks and it only uses the HTTP APIs. In this
case only, it’s safe to access data shared between txnp
and
txn_contp
without grabbing a lock. In the example above,
txn_contp
is created with a NULL
mutex. This works because the
HTTP transaction txnp
is the only one that will call back
txn_contp
, and you are guaranteed that txn_contp
will be called
back only one hook at a time. After processing is finished,
txn_contp
will reenable txnp
.
In all other cases, you should create a mutex with the continuation. In
general, a lock is needed when you’re using iocore APIs or any other API
where txn_contp
is scheduled to be called back by a processor (such
as the cache processor, the DNS processor, etc.). This ensures that
txn_contp
is called back sequentially and not simultaneously. In
other words, you need to ensure that txn_contp
will not be called
back by both txnp
and the cache processor at the same time, since
this will result in a situation wherein you’re executing two pieces of
code in conflict.
Special Case: Continuations Created for HTTP Transactions¶
If your plugin creates a new continuation for each HTTP transaction,
then you probably don’t need to create a new mutex for it because each
HTTP transaction (TSHttpTxn
object) already has its own mutex.
In the example below, it’s not necessary to specify a mutex for the
continuation created in txn_handler
:
static void
txn_handler (TSHttpTxn txnp, TSCont contp) {
TSCont newCont;
....
newCont = TSContCreate (newCont_handler, NULL);
//It's not necessary to create a new mutex for newCont.
...
TSHttpTxnReenable (txnp, TS_EVENT_HTTP_CONTINUE);
}
static int
test_plugin (TSCont contp, TSEvent event, void *edata) {
TSHttpTxn txnp = (TSHttpTxn) edata;
switch (event) {
case TS_EVENT_HTTP_READ_REQUEST_HDR:
txn_handler (txnp, contp);
return 0;
default:
break;
}
return 0;
}
The mutex functions are listed below: